Often thought of as the smallest unit of a living organism, a cell is made up of many even smaller parts, each with its own function. Human cells vary in size, but all are quite small. Even the largest, a fertilized egg, is too small to be seen with the naked eye.
Human cells have a surface membrane (called the cell membrane) that holds the contents together. However, this membrane is not just a sac. It has receptors that identify the cell to other cells. The receptors also react to substances produced in the body and to drugs taken into the body, selectively allowing these substances or drugs to enter and leave the cell (see Receptors on Cells). Reactions that take place at the receptors often alter or control a cell's functions. An example of this is when insulin binds to receptors on the cell membrane to maintain appropriate blood sugar levels and to allow glucose to enter cells.
Within the cell membrane are two major compartments:
- The cytoplasm
- The nucleus
The cytoplasm contains structures that consume and transform energy and perform the cell's functions.
The nucleus contains the cell's genetic material and the structures that control cell division and reproduction.
Mitochondria are tiny structures inside every cell that provide the cell with energy.
Inside a Cell
Although there are different types of cells, most cells have the same components. A cell consists of a nucleus and cytoplasm and is contained within the cell membrane, which regulates what passes in and out. The nucleus contains chromosomes, which are the cell's genetic material, and a nucleolus, which produces ribosomes. Ribosomes produce proteins, which are packaged by the Golgi apparatus so that they can leave the cell. The cytoplasm consists of a fluid material and organelles, which could be considered the cell's organs. The endoplasmic reticulum transports materials within the cell. Mitochondria generate energy for the cell's activities. Lysosomes contain enzymes that can break down particles entering the cell. Centrioles participate in cell division.
The body is composed of many different types of cells, each with its own structure and function. Some types of cells include
- Blood cells
- Muscle cells
- Skin cells
- Nerve cells
- Glandular cells
Some cells, such as blood cells, move freely in the blood and are not attached to each other. Other cells, such as muscle cells, are firmly attached to one another.
Some cells, such as skin cells, divide and reproduce quickly. Other cells, such as certain nerve cells, do not divide or reproduce except under unusual circumstances.
Epithelium: Surfaces of the Body
Some cells, especially glandular cells, have as their primary function the production of complex substances, such as a hormone or an enzyme. For example, some cells in the breast produce milk, some in the pancreas produce insulin, some in the lining of the lungs produce mucus, and some in the mouth produce saliva. Other cells have primary functions that are not related to the production of substances. For example, muscle cells contract, allowing movement. Nerve cells generate and conduct electrical impulses, allowing communication between the central nervous system (brain and spinal cord) and the rest of the body.
In addition to human cells, the human body has microorganisms that live on the skin and in the airways, mouth, and digestive, reproductive, and urinary tracts without causing any harm. The microorganisms that usually occupy a particular body site are called the resident flora, or the microbiome. Many resident flora are actually helpful to people—for example, by helping them digest food or by preventing the growth of other, more dangerous bacteria.